Optical device and method of converting wdm signals into an otdm signal and vice versa

ABSTRACT

A device and a method for converting WDM signals into an OTDM signal. A time shift is introduced between the pulses of the WDM signals carried by the optical carriers. A modulation means is ( 112, 113, 114 ) adapted to modify the optical power of the WDM signals, and an optical temporal multiplexer/demultiplexer ( 120 ) is provided. The WDM signals are injected into a birefringent propagation medium ( 130 ) in such a manner as to achieve a soliton trapping phenomenon. An absorption means ( 140 ) is adapted to introduce optical losses into the components of the OTDM signal. This technique performs WDM/OTDM conversion at very high bit rates. It also performs OTDM/WDM conversion. It is intended to be installed in long-haul telecommunication networks.

The present invention relates to an optical device and a method forconverting WDM (wavelength division multiplex) signals comprisingsimultaneous pulses carried by different wavelengths into an OTDM(optical time division multiplexing/demultiplexing) signal whosecomponents are time shifted and carried by a single wavelength, andvice-versa.

The field of the invention is that of optical telecommunications andmore particularly that of long-haul telecommunications. In the presentsituation of ever increasing bit rates on long-haul transmissionnetworks, increasing the transmission channel bit rate is inevitable,because it reduces the overall size, and more importantly the cost, ofthe terminal equipment. Accordingly, the next few years should see thedeployment on the transport networks of telecommunication operators ofthe first wavelength division multiplexing (WDM) plant operating at 40Gbit/s per wavelength and, in the longer term, at 160 Gbit/s perwavelength. This being the case, the requirements of transport networksin terms of optical time division multiplexing/demultiplexing (OTDM)will also increase. This being so, it is particularly beneficial toprovide an all-optical WDM/OTDM conversion function, in order to be ableto transfer the information conveyed by a plurality of wavelengths to asingle carrier, and an all-optical OTDM/WDM conversion function, inorder to be able to transfer to a plurality of optical carriers theinformation contained in an optical channel operating at a very high bitrate, typically at 40 Gbit/s, 160 Gbit/s or 640 Gbit/s. In the lattercase, the number of optical carriers involved in the conversion processis equal to the number of OTDM components of the optical signal to beconverted. These OTDM components may have a bit rate of 40 Gbit/s or 10Gbit/s.

Solutions for providing this kind of WDM/OTDM and OTDM/WDM conversionexist already, and include all-electronic solutions that useopto-electronic transponders equipped with photo receivers or laserdiodes to effect optical/electronic and electronic/optical conversion.Electronic components then handle time divisionmultiplexing/demultiplexing. Those solutions are complex to implement,however, because they require two-fold optical/electronic and/orelectronic/optical conversions and use a large number of components,which makes them difficult to install in the network because of obviousoverall size problems. They are also limited in terms of electricalbandwidth. The major drawback of those solutions is that their bit rateis limited because the electronics used are not able to operate at bitrates of 40 Gbit/s and above.

There also exist all-optical solutions in which OTDM/WDM conversionconsists in optical time division demultiplexing followed by wavelengthconversion. Optical time division demultiplexing uses crossed phasemodulation in a fiber, for example. That technology is very complex toimplement, however. The optical time division demultiplexing may also beeffected by means of non-linear optical mirrors using Mach-Zehnder,Michelson, or Sagnac interferometers. However, non-linear opticalmirrors have the drawback of being unstable; their stability is in facttemperature-dependent. Wavelength conversion is effected bysemiconductor optical amplifiers (SOA). A laser behind the SOA suppliesthe wavelength to which the signal must be converted. However, thatsolution uses a number of SOA and lasers equal to the number ofwavelength conversions to be effected, with the result that the cost ofthat solution is very high, which rules out large-scale deployment innetworks that are currently in full expansion. Moreover, SOAs are notcompletely bit rate transparent and distortion affecting the signal mayoccur.

WDM/OTDM conversion consists in converting the wavelength of each WDMsignal to a single wavelength and then carrying out optical timedivision multiplexing. Wavelength conversion again necessitates the useof a number of SOA and lasers equal to the number of WDM signals, withthe result that the cost of that solution is very high.

Finally, although the solutions described above for the two types ofconversion (OTDM/WDM and WDM/OTDM) have the advantage of beingall-optical solutions, which simplifies the signal processing system,they are able to operate only at low bit rates, below 40 Gbit/s.

Because of their limitations, the existing solutions cannot be used forWDM/OTDM or OTDM/WDM signal conversion at very high bit rates, that isto say bit rates above 40 Gbit/s.

Accordingly, the technical problem to be solved by the present inventionis that of proposing an optical device for converting WDM signals, thepulses of which are simultaneous and carried by different wavelengths,into an OTDM signal, the components of which are time shifted andcarried by the same wavelength, and enabling operation at very high bitrates, and thus enabling implementation in long-haul opticaltransmission networks operating at very high bit rates, typically of 40Gbit/s and above.

The solution according to the present invention of the technical problemas stated is obtained by the fact that said device comprises:

-   -   shifting means adapted to introduce a time shift between the        pulses of the WDM signals carried by the optical carriers,    -   modulation means adapted to modify the optical power of the WDM        signals,    -   an optical spectral and temporal multiplexer/demultiplexer,    -   a birefringent propagation medium into which the WDM signals are        injected in such a manner as to achieve a soliton trapping        phenomenon, and    -   absorption means adapted to introduce optical losses into the        components of the OTDM signal.

Thus the device of the invention uses the well-known phenomenon ofsoliton trapping in a birefringent propagation medium, which shifts theoptical frequency of the carrier in proportion to the optical power of asignal. By adjusting the optical power of the pulses of a signalbeforehand, soliton trapping shifts the wavelength of these pulsestoward a “target” wavelength of the optical carrier that is to carry theinformation.

The present invention solves the technical problem as stated byproviding a method of converting WDM signals, whose pulses aresimultaneous and carried by different wavelengths, into an OTDM signal,whose components are time shifted and carried by the same wavelength, bymeans of said device. This method is noteworthy in that it comprises thesteps of:

-   -   time shifting the pulses of the WDM signals carried by the        optical carriers,    -   attenuating the WDM signals in order for them to have different        optical powers,    -   spectrally and temporally multiplexing the WDM signals,    -   injecting the wavelength division multiplex obtained into the        birefringent propagation medium in such a manner as to achieve a        soliton trapping phenomenon and obtain an OTDM signal, and    -   equalizing the optical power of the components of the OTDM        signal obtained.

Another technical problem to be solved by the present invention is thatof proposing an optical device able to carry out the oppositeconversion, i.e. able to convert an OTDM signal, the components of whichare time shifted (t1, t2, t3, t4) and carried by the same wavelength(λ4), into WDM signals, the pulses of which are carried by differentwavelengths (λ1, λ2, λ3, λ4), and enabling operation at very high bitrates and implementation in long-haul optical transmission networks.

The solution according to the present invention of this problem isobtained by the fact that said device comprises:

-   -   absorption means adapted to introduce optical losses into the        components of the OTDM signal,    -   a birefringent propagation medium into which the OTDM signal is        injected in such a manner as to achieve a soliton trapping        phenomenon,    -   an optical spectral and temporal multiplexer/demultiplexer, and    -   modulation means adapted to modify the optical power of the WDM        signals.

The present invention solves this technical problem by providing amethod of converting an OTDM signal, whose components are time shiftedrelative to each other and carried by the same wavelength into WDMsignals, whose pulses are carried by different wavelengths, by means ofsaid device. This method is noteworthy in that it comprises the stepsof:

-   -   attenuating the components of the OTDM signal in such a manner        that they have different optical powers,    -   injecting the OTDM signal into the birefringent propagation        medium in such a manner as to achieve a soliton trapping        phenomenon and recover a wavelength division multiplex,    -   spectrally and temporally demultiplexing the wavelength division        multiplex in such a manner as to obtain a plurality of WDM        signals whose pulses are time shifted and carried by different        wavelengths, and    -   equalizing the optical power of the pulses of the WDM signals        obtained.

Other features and advantages of the invention will become apparent onreading the following description, which is given by way of illustrativeand non-limiting example and with reference to the appended drawings, inwhich:

FIG. 1 shows a device of the invention used as a WDM/OTDM converter,

FIG. 2 shows WDM signals at the input of the FIG. 1 device and at theoutput of the spectral and temporal multiplexer,

FIG. 3 shows signals at the input and at the output of the birefringentpropagation medium of the FIG. 1 device,

FIG. 4 shows absorption means used in the FIG. 1 device and signals atthe input and the output of the absorption means,

FIG. 5 shows different absorption means used in a different embodimentof the FIG. 1 device and signals at the input and output of thoseabsorption means, and

FIG. 6 shows a device of the invention used as an OTDM/WDM converter andsignals at each stage of conversion.

By way of example, the remainder of the description refers to convertingfour 40 Gbit/s WDM signals carried by four channels at differentwavelengths into a 160 Gbit/s OTDM signal carried by a single channel ona single optical carrier, and vice-versa.

The invention may of course be applied to signals having any bit rate.It is preferably applied to signals having bit rates of 40 Gbit/s, 160Gbit/s or even 640 Gbit/s.

The WDM/OTDM and OTDM/WDM conversion device is adapted to processsignals comprising RZ (return to zero) data which may be of the solitontype or a different type. An RZ signal is a digital signal comprising 0and 1 states, bits at 1 corresponding to pulses and bits at 0corresponding to the absence of any pulse in the bit period.

The device 100 in FIG. 1 is used as a WDM/OTDM converter. In thisexample, it is adapted to convert four 40 Gbit/s WDM signals carried byfour channels 10, 20, 30, 40 at different wavelengths λ1, λ2, λ3, λ4,for example, into a 160 Gbit/s OTDM signal carried by a single channelon a single optical carrier at the wavelength λ4.

There are shifting means 102, 103, 104 and modulation means 112, 113,114 at the output of the four WDM channels. The shifting means,consisting of delay lines, for example, introduce a time shift betweenthe pulses of the WDM signals carried by the optical carriers. Thisphase shift between the pulses is necessary for subsequent time divisionmultiplexing of the signals.

In this example, only three channels 20, 30, 40 are provided with thesedelay lines, since it is sufficient for each carrier to have a differentshift to the others. There is therefore no need to introduce atime-delay on the first channel 10, although there is nothing to rulethis out either, of course.

The delay lines 102, 103, 104 may be fixed and designed to shift eachoptical carrier by a fixed time period for each signal. It isnevertheless preferable to use variable delay lines in order to be ableto adjust and refine the shifts.

The optical modulation means 112, 113, 114 modulate the optical power ofthe WDM signals and comprise variable attenuators, for example.Accordingly, to attenuate them, different optical losses are induced ineach of the WDM signals, for example. There are then obtained WDMsignals carried by different wavelengths λ1, λ2, λ3, λ4 at differentoptical powers I1, I2, I3, I4 that are adjusted to achieve the solitontrapping effect required subsequently.

In this example, only three channels 20, 30, 40 are provided with theseattenuators, but each channel may be provided with an attenuator, forthe same reasons given for the delay lines. Variable optical attenuatorsare preferably used to adjust the power of each WDM signal.

In this example, the delay lines 102, 103, 104 precede the opticalattenuators 112, 113, 114, but in reality their order is of noimportance at this stage. It is sufficient if the WDM signals at theinput of the optical multiplexer/demultiplexer 120 have been shifted andmodulated.

The optical spectral and time division multiplexer/demultiplexer 120then multiplexes the WDM signals so that there is only one wavelengthdivision multiplex comprising time shifted (t1, t2, t3, t4) pulses atdifferent wavelengths λ1, λ2, λ3, λ4 and different powers I1, I2, I3,I4.

The multiplex obtained in this way is then injected into birefringentpropagation means 130, for example a birefringent optical fiber, toproduce the soliton trapping phenomenon and obtain a time divisionmultiplex signal constituting an OTDM signal carried by a singlewavelength, the wavelength λ4 in this example.

Absorption means 140 then equalize the optical powers of the componentsof the final OTDM signal.

FIGS. 2 to 5, being more detailed, enable the operation of this deviceduring WDM/OTDM conversion to be explained more clearly.

FIG. 2 shows the timing diagram of each WDM signal at the input of thedevice and that of the wavelength division multiplex at the output ofthe optical spectral and time division multiplexer/demultiplexer 120. Atthe input of the device, each WDM signal comprises pulses carried by adifferent wavelength λ1, λ2, λ3, λ4. The pulses of the various WDMsignals all have the same intensity I1 and occur simultaneously.

At the output of the multiplexer 120, the multiplex comprises timeshifted (t1, t2, t3, t4) pulses at different wavelengths λ1, λ2, λ3, λ4and different intensities I1, I2, I3, I4.

The pulses of the OTDM signal to be obtained at the output of the devicemust be interleaved. The shift between two pulses must therefore beidentical each time. Accordingly, at 160 Gbit/s, for example, the pulsesare shifted relative to each other by 6.25 ps. The shift between thepulses is therefore set and adjusted beforehand by means of the delaylines 102, 103, 104.

The optical power I1, I2, I3, I4 of each pulse of the wavelengthdivision multiplex is adjusted beforehand by means of variableattenuators 112, 113, 114 to exacerbate non-linear effects in thebirefringent optical fiber 130 and thereby encourage the requiredsoliton trapping effect, as shown in FIG. 3.

A birefringent propagation medium comprises two main propagation axes.To encourage the soliton trapping phenomenon, the multiplex is injectedwith a polarization at 45° to the main propagation axes of thebirefringent medium 130. In this case, a polarization controller mayprecede the optical fiber 130, for example, to convert any incomingpolarization to another polarization and in particular a linearpolarization at 45° to the main axes of the birefringent fiber.

A soliton is a light pulse that is sufficiently intense to excite anon-linear effect that compensates the effects of chromatic dispersionover long distances. Under some conditions, in particular conditions ofpower and chromatic dispersion well known to the person skilled in theart, the injected pulses 1 to 4 retain their integrity and are nottemporally deformed. Their frequency spectrum is deformed, however, anda frequency shift relative to the original frequency of the spectrum ofeach of these pulses occurs on entering the propagation medium. Thisphenomenon, during the course of which the pulse is not temporallydeformed but the spectrum suffers a frequency shift, is known as solitontrapping. The frequency shift ΔυI is proportional to the luminous powerIi of the pulse i injected into the propagation medium.

Accordingly, by precisely adjusting the luminous power Ii of each pulsei of the wavelength division multiplex, the frequency shift ΔυI inducedby the soliton trapping phenomenon in the pulse i of the wavelengthdivision multiplex may be adjusted to achieve perfect spectral matchingof the spectrum shifts of the WDM channels. This precise adjustment isobtained by means of the variable delay lines and the variableattenuators preceding the multiplexer 120. In the FIG. 3 example, thepulses 1, 2, 3 with respective intensities I1, I2, I3 are subjected toshifts Δυ1, Δυ2, Δυ3 so that their wavelengths all coincide with thewavelength λ4 of the fourth pulse.

An OTDM signal is therefore obtained at the exit from the birefringentmedium whose components are time shifted (t1, t2, t3, t4) and carried bya single wavelength λ4.

However, the components of the OTDM signal obtained do not have the sameluminous power I1, I2, I3, I4. Absorption means 140 are thereforeprovided to return all the components of the OTDM signal to the sameoptical power level.

This power equalization uses an electro-absorption modulator MEA, forexample, that applies selective optical losses to the components of theOTDM channel, as shown in FIG. 4. The losses Pos may have a stepped timeprofile (142) or a linear ramp time profile (143), as shown by thecurves of the applied voltage V and the output optical losses Pos as afunction of time t. The continuous line curve relates to the appliedvoltage V and the dashed line curve relates to the output optical lossesPos.

Accordingly, the absorption of the MEA being a function of the appliedvoltage V and time, the components of the injected signal are notsubject to the same absorption on entering the MEA because each has adifferent intensity and they are time shifted relative to each other.The components 1, 2, 3, 4 then have exactly the same optical power Is atthe output of the MEA.

A different embodiment uses a saturable absorber to effect this powerequalization, as shown in FIG. 5. The transfer function of a saturableabsorber comprises two different states, namely a blocking state whenthe input power Ie is below a threshold power It and a totallytransparent state when the input power is above that threshold power. Inthe transparent state, the output signal of the saturable absorber has aconstant output power Is. If the components of the OTDM signal obtainedall have powers I1, I2, I3, I4 above the threshold power It, they allhave the same output power Is at the output of the absorber. If,however, the components of the OTDM signal have a power below thethreshold power, they are totally absorbed.

The device 100 may also be used to carry out the opposite conversion,i.e. to convert an OTDM signal into WDM signals. This conversion usesthe same device in the opposite direction. It is therefore describedmore succinctly, and with reference to FIG. 6, which shows the deviceused as an OTDM/WDM converter and the signals at each stage of theconversion.

The OTDM signal is initially passed through absorption means 140 inorder for selective optical losses to be applied to its components. Theabsorption means comprise the electro-absorbent modulator MEA describedabove, for example. The components of the OTDM signal do not suffer thesame absorption and therefore suffer different optical losses.

The OTDM signal obtained is then injected into the birefringent opticalfiber 130 to achieve the soliton trapping effect described above. Inthis case, the components of the OTDM spectrum are subjected to afrequency shift ΔυI proportional to their optical power. A wavelengthdivision multiplex is therefore obtained whose pulses 1, 2, 3, 4 aretime shifted relative to each other and carried by different wavelengthsλ1, λ2, λ3, λ4 and have different optical powers I1, I2, I3, I4.

Just as for WDM/OTDM conversion, a polarization controller may precedethe optical fiber 130, for example, to facilitate injection of thesignal with a polarization at 45° to the main axes of the optical fiber.

The next step is to pass the wavelength division multiplex through themultiplexer/demultiplexer 120 in order to demultiplex it spectrally andtemporally and to obtain four signals at different wavelengths λ1, λ2,λ3, λ4.

The final step is to equalize the optical powers of the pulses of theWDM signals. This is effected by the modulation means 112, 113, 114,which comprise variable attenuators as described above, for example.

It is not essential to use the FIG. 1 shifting means 102 to 104 for theOTDM/WDM conversion. If those means, for example delay lines, are used,they time shift the pulses carried by the optical carriers of the WDMsignals in such a manner as to render them simultaneous.

The device that has just been described is no more than an illustrationof the invention, which is in no way limited to this example and hasapplications in high bit rate long-haul optical telecommunications.

The device has the advantage of being all optical and is easy toimplement and to install in the network. It uses no laser sources, onlylow-cost components, and is independent of bandwidth. Finally, thedevice is of very great benefit for the incoming generations of high bitrate transmission systems operating at bit rates of 40 Gbit/s and above.

1. An optical device (100) for converting WDM signals, the pulses ofwhich are simultaneous and carried by different wavelengths (λ1, λ2, λ3,λ4), into an OTDM signal, the components of which are carried by thesame wavelength (λ4) and time shifted (t1, t2, t3, t4), which devicecomprises: shifting means (102, 103, 104) adapted to introduce a timeshift between the pulses of the WDM signals carried by the opticalcarriers, modulation means (112, 113, 114) adapted to modify the opticalpower of the WDM signals, an optical spectral and temporalmultiplexer/demultiplexer (120), a birefringent propagation medium (130)into which the WDM signals are injected in such a manner as to achieve asoliton trapping phenomenon, and absorption means (140) adapted tointroduce optical losses into the components of the OTDM signal.
 2. Anoptical device for converting an OTDM signal whose components are timeshifted (t1, t2, t3, t4) and carried by the same wavelength (λ4) intoWDM signals whose pulses are carried by different wavelengths (λ1, λ2,λ3, λ4), which device comprises: absorption means (140) adapted tointroduce optical losses into the components of the OTDM signal, abirefringent propagation medium (130) into which the OTDM signal isinjected in such a manner as to achieve a soliton trapping phenomenon,an optical spectral and temporal multiplexer/demultiplexer (120), andmodulation means (112, 113, 114) adapted to modify the optical power ofthe WDM signals.
 3. A device according to claim 2, characterized in thatit further comprises shifting means (102, 103, 104) adapted to introducea time shift between the pulses of the WDM signals carried by theoptical carriers.
 4. A device according to claim 1 or 2, characterizedin that the shifting means (102, 103, 104) comprise variable delaylines.
 5. A device according to claim 1 or 2, characterized in themodulation means (112, 113, 114) comprise variable attenuators.
 6. Adevice according to claim 1 or 2, characterized in that it furthercomprises a polarization controller at the entry of the birefringentpropagation medium (130) to encourage the injection of WDM/OTDM signalsinto said propagation medium with a polarization at 45° to its mainaxes.
 7. A device according to claim 1 or 2, characterized in that theabsorption means (140) comprise an electro-absorption modulator (MEA).8. A device according to claim 1 or 2, characterized in that theabsorption means (140) comprise a saturable absorber.
 9. A method ofconverting WDM signals, the pulses of which are simultaneous and carriedby different wavelengths (λ1, λ2, λ3, λ4), into an OTDM signal, thecomponents of which are time shifted and carried by the same wavelength(λ4), by means of the device according to claim 1 or 2, which method iscomprises the steps of: time shifting the pulses of the WDM signalscarried by the optical carriers, attenuating the WDM signals in orderfor them to have different optical powers, spectrally and temporallymultiplexing the WDM signals, injecting the wavelength divisionmultiplex obtained into the birefringent propagation medium in such amanner as to achieve a soliton trapping phenomenon and obtain an OTDMsignal, and equalizing the optical power of the components of the OTDMsignal obtained.
 10. A method of converting an OTDM signal, thecomponents of which are time shifted (t1, t2, t3, t4) and carried by thesame wavelength (λ4) into WDM signals, the pulses of which are carriedby different wavelengths (λ1, λ2, λ3, λ4), by means of the deviceaccording to claim 2, which method comprises the steps of: attenuatingthe components of the OTDM signal in such a manner that they havedifferent optical powers, injecting the OTDM signal into thebirefringent propagation medium in such a manner as to achieve a solitontrapping phenomenon and recover a wavelength division multiplex,spectrally and temporally demultiplexing the wavelength divisionmultiplex in such a manner as to obtain a plurality of WDM signals whosepulses are time shifted and carried by different wavelengths, andequalizing the optical power of the pulses of the WDM signals obtained.11. A method according to claim 10, characterized in that it furtherconsists in time shifting the pulses of the WDM signals carried by theresulting optical carriers in such a manner as to render themsimultaneous.